It is conventional to grow single crystal silicon ingots by preparing a melt of the silicon material, and contacting the surface of the melt with a previously prepared seed crystal of the silicon material of the desired crystalline lattice orientation. The seed crystal is then withdrawn from the melt at a rate of approximately a few inches per hour, while the crystal and the melt are counter-rotated with respect to each other.
Typically, the chamber in which the crystal ingot is grown is first evacuated and then backfilled to a lower than atmospheric pressure with a continuing flow of gas, such as argon, which serves as the ambient during the crystal growth. The lower than atmospheric pressure of the gas aids in minimizing the formation of undesired contaminants in the system during the growth. With this technique, commonly known as the Czochralski process, crystals several feet in length and several inches in diameter are routinely grown.
Oxygen in the semiconductor crystals is essential for internal gettering of impurities upon precipitation (e.g., SiO.sub.2) during device processing. The source of the oxygen is the fused silica crucible used to contain the silicon melt in the Czochralski process. The incorporation level of oxygen in silicon is a function of, among other things, the crucible dissolution rate. Different levels of oxygen concentrations are required for different device processing, depending on the nature of the particular device fabrication process. For example, bipolar processing requires higher oxygen concentrations than MOS processing. Control of crucible dissolution by the crucible rotation rate is an effective method of controlling the oxygen level in the melt. Accelerated and decelerated crucible rotation is effective to enhance oxygen incorporation in many cases. Such motion increases the mass transfer between the wetted walls of the crucible and the melt. Therefore, the available wetted inner surface area of the crucible and the relative motion between the crucible and the melt are important factors associated with oxygen incorporation. At present, using the ASTM-80 standard, the amount of oxygen incorporation is in the range of 12-18 ppma with the bulk of the crystal ingot grown in the 12-16 ppma range However, certain material codes require a higher range of oxygen content, for example, 17-22 ppma Therefore, there is a need for substantially increasing the level of oxygen incorporation in the grown crystal billet.